GENBIO Chapter 16 PDF
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University of Santo Tomas
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This document covers the chapter on Nucleic Acids and Inheritance. It goes into detail about DNA replication, the experiments surrounding DNA transfer, protein, and the role of DNA replication in cell processes. It provides basic biology definitions and concepts related to genetic information transfer.
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Nucleic Acids and Inheritance ★ T 2 attaches to host cell; injects ❖ DNA replicationallows genetic info genetic material through plasma...
Nucleic Acids and Inheritance ★ T 2 attaches to host cell; injects ❖ DNA replicationallows genetic info genetic material through plasma to beinheritedfrom generation to membrane; head and tail outside generation Unduplicated chromosome → DNA Alfred Hershey and Martha Chase segment from chromosome → Replication ❖ DNA is the genetic material of T2 begins at multiple origins, forming a T2 infects E.coli replication bubble→ Duplicated and Could turn an E.coli cell into condensed chromosome → Two DNA a T2-producing factory molecules (daughter cells) Reprograms its host cell to ★ Eachgeneisunit of hereditary produce viruses informationw/ aspecific DNA ❖ T2 enters E.coli cell sequence ❖ Found that the phage DNA entered ★ Initially, it was believed thatproteins the host cells but phage protein did carried genetic information not DNA played an ongoing role Frederick Griffith during the infection process ❖ Studiedstreptococcus pneumoniae DNAcarries the genetic (causes pneumonia) to develop a information vaccine Experiment in steps… ❖ Mixed remains of pathogenic Batch I: Radioactive sulfur for protein bacteria with non pathogenic Batch II: Radioactive phosphorus for DNA bacteria, whichcaused the non 1. Mixedradioactively labeled phages pathogenic to become pathogenic with bacteria Transformation 2. Agitatedthe mixture in blender to ❖ Change in genotype and free phage parts phenotypedue toassimilation of 3. Centrifugedthe mixture so that external DNAby a cell bacteria formed a pellet at the ❖ The transforming substance was bottom; phages and phage parts DNA remained suspended in the liquid 4. Measuredthe radioactivity Evidence that DNA can program cells: Results & Conclusions: ❖ Bacteriophagesorphages ❖ Proteins were labeled, radioactivity Used as tools by researchers remained outside the cells viruses thatinfect bacteria ❖ DNA were labeled, radioactivity was ❖ Viruses found inside the cells Much simpler than cells ❖ Phage DNA entered bacterial cells Little more than DNA or RNA but phage proteins did not enclosed by a protective coat (protein) ther Notable Experiments: O Infect a cell and take over the Erwin Chargaff cell’s metabolic machinery ❖ DNA known to be polymer of nucleotides C ontains a nitrogenous base, ❖ W atson constructed a model in deoxyribose sugar, and which the two sugar phosphate phosphate group backbones areantiparallel Adenine, thymine, guanine, Subunits run in opposite or cytosine directions ❖ Analyzed thebase compositionof ❖ Rope ladder with rigid rungs DNA Side ladder (sugar Variesfrom one species to phosphate backbones) another Rungs (nitrogenous bases) ❖ Molecular diversityamong species Twist the ladder to form a ❖ Regularity in the ratios of nucleotide helix bases ❖ One full turn every 3.4nm along its Adenines approximately length; bases stacked 0.34nm apart equaled the number of Ten layers of base pairs in thymines, and guanines with each full turn of the helix cytosines ❖ Nitrogen bases are paired in specific Chargaff’s rules combinations (A&T; G&C) 1. DNAbase composition varies Adenine and Guanine are between species purines; two organic rings 2. For each species,A and T base Cytosine and Thymine are percentages are roughly equal, pyrimidines; single ring same with G and C bases Possible pairings ★ Basis of these rules remained 1. Purine + Purine= toowide unexplained until the discovery of 2. Pyrimidine + Pyrimidine= too thedouble helix narrow 3. Purine + Pyrimidine= width Structure of a DNA strand consistent with X-ray data ❖ Deoxyribose sugar, nitrogenous ❖ Adenine can form two hydrogen base, and phosphate group bonds with thymine; Guanine can ❖ Phosphate group attached to sugar, form three hydrogen bonds with forming a “backbone” Cytosine ❖ 5’ (phosphate group) and 3’ (sugar) end Base pairing in DNA ★ Rosalind Franklin (backbone on the ❖ Nitrogenous base pairs held outside), Linus Pauling (three together byhydrogen bonds stranded), Maurice Wilkins attempted to find the three ❖ T he Watson-Crick Model explained dimensional structure of DNA Chargaff’s ratios First to answer… ❖ Whenever one strand of DNA has an James Watson and Francis Crick A, its partner has a T (same for ❖ Discovered that DNA washelicalin G&C) shape Explains why the amount of ❖ Helix made up of two strands adenine equals the amount Double helix of thymine etc. ❖ B ase pairing rulesdo not restrict the ❖ S equence of the pairs of bases will sequence of nucleotidesalong each have been duplicated exactly DNA strand ❖ Two strands arecomplementary; Varied in countless ways each stores the information Each gene has aunique necessary to reconstruct the other base sequence ❖ Nucleotides line up according to the ❖ Nobel Prize for double helix idea base pairing rules ❖ Structure of DNA suggested the Linked to form new strands basic mechanism of its replication ❖ One double-stranded DNA molecule becomes two, each anexact replica any proteins work together inDNA M of the parental molecule replicationand repair ❖ Hereditary informationdirects traits Models of Replication ❖ Resemblance has its basis in ❖ When adouble helixreplicates, accurate replication of DNA prior to each of the two daughter molecules meiosis will haveone old strand, and one ❖ Replicationensures faithful newly made strand transmission of genetic information (semi-conservative model) ❖ Nucleic acids areunique in their ❖ Two parental strands come back ability to dictate replication together after the process ❖ Specific complementary pairing of (Conservative) nitrogenous bases has a functional ❖ All four strands of DNA have a significance mixture of old and new DNA (Dispersive) he Basic Principle: Base Pairing to a T Template Strand (Overview) Matthew Meselson and Franklin Stahl Steps… ❖ Devised an experiment to distinguish 1. Parental molecule hastwo the three models complementary strandsof DNA ❖ Results supported the 2. Two strands areseparated and semiconservative model of DNA serve as a templatefor a new strand replication 3. Nucleotides complementaryto the The Experiment in Steps: parental strand areconnectedto 1. Bacteria cultured with heavy isotope form the backbones of the new 2. Transferred with lighter isotope daughter strands 3. DNA sample centrifuged after first replication ❖ H ydrogen bonds are broken, 4. DNA sample centrifuged after causing the two chains to unwind second replication and separate Conclusion ❖ Each strand acts as a template for ❖ First replication produced a band of the formation of a new companion many molecules of a hybrid chain Eliminated the conservative ❖ Two pairs of chains from the one model before ❖ S econd replication produced both ❖ T he parental strands separate there light and hybrid DNA and form areplication bubble with Eliminated the dispersive two forks model ❖ Replication proceeds in both directions until the forks meet on the NA Replication: A closer look D other side Origins of replication ❖ Replication of DNA begins at these . E b ukaryotes sites ❖ Replication bubbles form at many ❖ Short stretches of DNA that have a sites along the giant DNA molecule specific sequence of nucleotides during S phase ❖ Bubbles expand as replication Replication proceed in both directions ❖ Proteins that initiate DNA replication ❖ Bubbles fuseand synthesis of the attach to the DNA and separates the daughter cells is complete two strands, opening up a replication bubble Synthesizing a New DNA strand ❖ The replication then proceeds in ❖ DNA polymerasesrequire a primer both directions to which they can add nucleotides ❖ Eukaryotic chromosomes may have ❖ Theinitial nucleotide chainis a short hundreds or even a few thousand RNAprimer replication origins ❖ This is synthesized by the enzyme Multiple replication bubbles primase form and fuse, speeding up ❖ Completed primer is five to ten the copying process nucleotides long ❖ At the end of each replication bubble ❖ New DNA strand will start from the3’ is areplication fork,aY-shaped endof the RNA primer region where parental DNA strands ❖ DNA polymerasescatalyze the are being unwound synthesis of new DNA byadding ❖ Helicasesare enzymes thatuntwist nucleotides to the 3’ endof a the double helixat the replication pre-existing chain forks Catalyzes theaddition of ❖ Single-strand binding proteins each monomerto the bind to and stabilize single-stranded growing end of a DNA strand DNA by acondensation reactionin ❖ Topoisomeraserelieves the strain which two phosphate groups of twisting of the double helixby are lost breaking, swiveling, and rejoining DNA polymerase IIIadds a DNA strands DNA nucleotide to the RNA primerthen continues adding rigins of replication in E.coli and O them eukaryotes DNA polymerase III a. E.coli 1. DNA pol III starts to synthesize the ❖ Onlyone origin of replication leading strand 2. C ontinuous elongation in the 5’ to 3’ 4. P rimasesynthesizes an RNA primer direction for the next Okazaki fragment. 5. DNA polymerase IIIcompletes the ❖ D NA polymerases can add synthesis of the previous fragment nucleotidesonly to the free 3’ endof and detaches to begin adding a primer nucleotides to the next fragment. ❖ Elongate only in the 5’ → 3’ 6. DNA polymerase Iremoves RNA Creates aleading strand primers and replaces them with DNA Only needs1 primer(serves on the lagging strand. as the starting point for DNA 7. DNA ligasejoins the fragments synthesis) together by sealing the gaps ❖ To elongate the other new strand, thelagging strand, DNA Trombone model polymerase must work in the ❖ Two DNA polymerase molecules directionaway from the replication reel in the parental DNA and extrude fork newly made daughter DNA ❖ The lagging strand is synthesized as molecules a series of segmentscalledOkazaki ❖ Two molecules of DNA pol III work fragments, which are joined together in a complex one on each together byDNA ligase strand with helicase and other Steps of the synthesis of the lagging strand proteins 1. Primasecreates an RNA primer to ❖ Lagging strand template DNA loops start DNA synthesis. through the complex 2. DNA polymerase IIIadds DNA nucleotides, forming Okazaki acterial DNA replication Proteins and their B fragments. functions 3. DNA polymerase IIIdetaches at the Helicase next RNA primer. ❖ Unwindsparental double helix at 4. A new RNA primer starts the next replication forks fragment, extended by DNA polymerase III. Single-strand binding protein 5. DNA polymerase Ireplaces RNA ❖ Binds to andstabilizes primers with DNA. single-stranded DNAuntil it is used 6. DNA ligasejoins fragments into a as a template continuous strand. Topoisomerase Bacterial DNA replication ❖ Relievesoverwinding strain 1. Helicaseunwinds the parental DNA double helix. Primase 2. Single-strand binding proteins ❖ Synthesizes an RNA primerat 5’ stabilize the separated strands. end of leading strand and at 5’ end 3. Theleading strandis synthesized of each Okazaki fragment of lagging continuously by DNA polymerase in strand the 5' to 3' direction. DNA pol III ❖ S egment of thestrand containing the ❖ Using parental DNA as a template, damage is cut out synthesizes new DNA strand by Gap filled adding nucleotides to an RNA primer or a pre-existing DNA strand DNA repair systems: 1. Enzymes detect and repair DNA DNA pol I damage that distorts the molecule. ❖ Removes RNA nucleotidesof primer 2. The nuclease enzyme cuts the from 5’ end and replaces them with damaged DNA strand at two points nucleotides added to 3’ end of and removes the damaged section. adjacent fragment 3. DNA polymerase synthesizes new nucleotides to fill in the missing part, DNA ligase using the undamaged strand as a ❖ Joins Okazaki fragmentsof lagging template. strand; on leading strand,joins 3’ 4. DNA ligase seals the newly endof DNA that replaces primer to synthesized strand to the existing rest of leading strand DNA DNA, completing the repair. ❖ U pon finding an incorrectly paired Mutation nucleotide, polymerase removes the ❖ Permanent changein the DNA nucleotide then resumes synthesis sequence ❖ Changesthephenotypeof an DNA polymerases organism ❖ Catalyze the synthesis of new DNA at replication fork hortening of the ends of linear DNA S ❖ Most DNA polymerases require a molecules primer and a DNA template strand ❖ After the first round, newlagging ❖ The rate of elongation is about 500 strand is shorterthan its template nucleotides per second in bacteria ❖ After the second round, both the and 50 per second in human cells leading and lagging strands are shorterthan the original parental Mismatch repair DNA ❖ Other enzymesremove and replace incorrectly paired nucleotidesthat Telomeres have resulted from replication errors ❖ Special nucleotide sequencesat the Errors may arise after endsof Eukaryotic chromosomal replication DNA Changes usually corrected ❖ Do not present the shortening of before they become DNA molecules, but they do mutations postpone the erosion of genesnear the ends of DNA molecules Nuclease ❖ Proposed that the shortening of ❖ DNA-cuttingenzyme telomeres is connected to aging ❖ M ultiple repetitions of one short nucleotide sequence hromosomeconsists of aDNA molecule C packed together with proteins Histones ❖ Responsible for the main level of DNA packingin interphase chromatin ❖ Bind tightly to each otherand to the DNA to formnucleosomes Nucleosomes ❖ Basic unit of DNA packing ❖ The string between them is called linker DNA Chromatin ❖ Complex of DNA and protein ❖ Prophase (begins to condense) ❖ Prometaphase ❖ Metaphase (most dense) Euchromatin ❖ Less compacted, more dispersed interphase chromatin Heterochromatin ❖ More compacted, denser
